Water-soluble unbleached bag paper, and paper bag

ABSTRACT

In a water-soluble unbleached bag paper comprising lignin-containing cellulose fibers as main component and one or more additive(s), at least one surfactant is contained as additive, and the bag paper has a relative wet strength of less than 6% after a 5-minute wetting time. Furthermore, a paper bag comprised of water-soluble unbleached bag paper is described.

The present invention relates to a water-soluble unbleached bag papercomprising lignin-containing cellulose fibers as main component and oneor more additive(s), and a paper bag comprised of water-solubleunbleached bag paper.

Bag paper for the production of paper bags and sacks for variouscontents usually involves high demands on the mechanical tensileproperties of the paper both in the machine direction and transversethereto in order to prevent inadvertent tearing of the paper, whetherduring filling, stacking, storing or transport, and in addition to goodtensile properties it should, moreover, exhibit high porosity andsufficient extensibility both in the long and transverse directions.Porosity is important to avoid a pressure jam in the interior of a bag,for instance when filling the bag, which usually occurs by rapidintroduction using machines, and hence tearing of the bag due to a“pressure wave” and, moreover, to ensure that the dust load caused bythe filling of powdery materials like cement or the like will be kept aslow as possible.

Such a bag paper has been known for quite some time and produced indifferent qualities for a multitude of applications. Aside from theabove-mentioned properties of the bag paper, bag paper has beenavailable in both bleached and unbleached form, wherein bleached bagpaper has a considerably lower lignin content than unbleached bag paper,which makes such bag papers perfectly suitable for recycling purposesand, in particular, for instance dissolving in water or water withadditives while applying shearing forces. Unbleached bag papers, whichhave high contents of lignin, are usually not or hardly water-solublebecause lignin is a naturally occurring hydrophobing agent, andtherefore constitute a relatively big problem, in particular withrespect to their disposal. Disposal or recycling by the dissolution ofsuch papers is presently not possible because of the poor dissolutionproperties, and the introduction of such bags, for instance, into wasteincineration would raise problems insofar as, for instance in the eventof cement, sacks, residual amounts of raw material or filler materialremaining in the bags would eventually cause a concentration of, orincrease in, cement or burnt lime or the like in combustion furnaces,which might, in turn, involve problems of removal and the obstruction ofimportant furnace elements.

Water-soluble papers, in contrast, do not show such problems and thuscan be disposed of at once. Water-soluble papers, in particular bagpapers, are characterized by the complete dissolubility of the paperafter treatment, according to norm ISO 5263 “Pulps—Laboratory WetDisintegration”, and a water penetration rate into the paper determinedby a K-value that must be >5000. The R-value in this context is measuredby an Emco penetration tester DPM 27. K-value measurements take place atroom temperature from 25° C.±2° C. with pure cold water (drinking waterquality) having a temperature between 15° C. and 20° C. and no additiveswhatsoever.

The present invention aims to provide a water-soluble unbleached bagpaper that is completely soluble in water according to ISO 5263 and,moreover, comprises a water penetration rate into the paper comparableto that of bleached bag paper, without the cellulose fibers having to besubmitted to a bleaching process prior to their use.

To solve this object, the water-soluble unbleached bag paper accordingto the invention is essentially characterized in that at least onesurfactant is contained as said additive and that the bag paper has arelative wet strength of less than 6% after a 5-minute wetting time. Theaddition of a surfactant allows for a reduction of the surface tensionof the thus produced bag paper to such an extent that the effect oflignin as hydrophobing agent will be compensated for, thus rendering thepaper water-soluble to the point that it will be dissolved within a fewminutes, preferably about 5 minutes, and nothing but cellulose fiberswill be detectable. Moreover, the addition of the surfactant does notaffect the overall strength of the paper such that the properties of thebag paper are comparable to those of a non-water-soluble unbleached bagpaper without having to perform a complex cellulose fiber regenerationprocedure for removing the lignin, i.e. a bleaching process, so as tomake available a paper that can, in particular, be economicallyproduced.

In that the bag paper has a relative wet strength of less than 6% aftera 5-minute wetting time, it has become possible to dissolve the bagpaper nearly completely within a short time so as to enable such a bagpaper to be, for instance, directly introduced into a concrete mixeralong with the construction materials contained therein, e.g. cement,for instance free from residues, and subjected to a mixing process, thusrendering possible the completely residue-free processing of such a bagpaper without the thus produced concrete having, for instance,deteriorated properties.

Just by way of comparison, it should be noted that known papers that arealso substantially water-soluble, such as toilet paper, which is madefrom fresh fibers, have wet strengths of about 8% after a 5-minutewetting time, which implies that the paper according to the inventiondissolves particularly rapidly and completely.

According to a further development of the invention, the bag paper isdevised such that between 0.1% and 2% surfactant is contained. Theamount of surfactant in this respect is, in particular, selected suchthat, on the one hand, the surface tension will not be reduced too muchin order to prevent the unlimited and instantaneous water absorption ofthe paper. Too rapid and too complete a water absorbency of paper, forinstance, involves the drawback of such a bag paper being not printable,which is, in particular, disadvantageous for papers and bags used inindustrial applications, since, for instance, in the constructionindustry the content of a bag is usually directly printed on the same.If the absorption of water is too high, such papers will actually be toohygroscopic for flexoprinting, which is usually employed for printingbag papers, and either the paper will not be printable at all or theprint will leak or bleed, which makes such a paper undesirable forindustrial use.

According to a further development of the invention, the additive of thebag paper is selected such that the at least one surfactant is selectedfrom the group of non-ionic, anionic, cationic or amphotericsurfactants, fatty alcohols, fatty alcohol polylglycol ethers, fattyalcohol sulfates, fatty alcohol ether sulfates, methyl ether sulfates,fatty alcohol ethoxylates or phenol ethoxylates. What type of surfactantis selected for a specific purpose of use has turned out to benoncritical and is to be done as a function of the lignin content of theemployed cellulose fibers, the type of wood from which the cellulosefibers are derived, the surface tension of the bag paper and the like,so that basically any of the group of ionic or non-ionic, anionic,cationic or amphoteric surfactants, yet also fatty alcohols, fattyalcohol polylglycol ethers, fatty alcohol sulfates, fatty alcohol ethersulfates, methyl ether sulfates, fatty alcohol ethoxylates or phenolethoxylates may be chosen.

The paper according to the invention has an absorbency corresponding toa K-value of more than 5000.

The K-value is a measure for the penetration rate of water into paper,and hence the absorbency of the paper. It is usually measured byultrasonic transmission. During such measurement, the ultrasonictransmitter and the ultrasonic receiver are arranged within a measuringcell in such a manner that the sound wave will directly pass through thepaper sample to be checked. During measuring, an ultrasonic wave with aconstant frequency and intensity is generated on the transmitter. Theinteraction of the paper sample with the surrounding medium, i.e. water,causes the ultrasonic intensity to change on the receiver, and this isplotted in a transmission time diagram. The determined K-value will thenindicate the gradient of the curve at the time of the highestpenetration rate. The measuring procedure applied here is the measuringprocedure according to the Emco penetration tester DPM 27.

In order to increase the strength of the bag paper, it can be proceededaccording to the invention such that a dry-strength agent isadditionally contained. According to the present invention, starcheslike cationic potato starch, corn starch, tapioca starch are used asdry-strength agents. The printability of the paper will be enhanced bythe addition of dry-strength agents.

According to development of the invention, the cellulose fibers used foran unbleached, water-soluble bag paper according to the invention areunbleached softwood fibers. Furthermore, softwood fibers generally showa higher fiber length in comparison to fibers extracted from hartwood,so that by the fibers interlocking among others, a higher strength ofthe bag paper can be achieved just by use of special fibers, without theneed for adding a dry-strength agent or respectively only a very smallamount of dry-strength agent would be needed.

According to a further development of the invention, the cellulosefibers are selected from softwood fibers from conifers, in particularfibers from spruce wood and/or pinewood.

To adjust the desired strength, it is proceeded, as in correspondencewith a further development of the invention, such that the softwoodfibers are used, as high-consistency-beaten (HC-beaten) and/orlow-consistency-beaten (LC-beaten) softwood fibers. Depending on theapplied beating procedure, the fiber length of the used cellulose ismore or less increased, which will also influence the strength of thethus produced, paper. According to the present invention, eitherhigh-consistency-beaten softwood fibers or low-consistency-beatensoftwood fibers can be used exclusively or in mixtures thereof.

In order to achieve a stable bag paper that is sufficiently strong, inparticular in the dry state, the soaked bag is configured such that thelongitudinal/transverse ratio of the tensile strength is between 1 and1.8, in particular 1.3. Bag papers are produced on so-called Clupakunits, which enable the production of papers having nearly identicaltensile strength properties both in the longitudinal and in thetransverse directions. The more uniform the tensile strength propertiesin the longitudinal and transverse directions, the more stable and moresuitable a thus produced paper will be for practical applications, whereup to 50 kg cement or more are, for instance, filled into such a bag,and which bag must by no means tear during or after filling. Thewater-soluble unbleached bag paper according to the invention enablesthe achievement of a longitudinal/transverse ratio of between 1 and 1.8,in particular 1.3.

In order to, in particular, achieve a good longitudinal/transverse ratioand produce a bag paper having excellent strength properties at asimultaneous water solubility, it is proceeded according to the presentinvention such that the cellulose fibers are contained as dried, inparticular flake-dried, pulp.

The present invention also relates to a paper bag comprised of thewater-soluble unbleached bag paper according to the invention. Such abag is essentially characterized in that the bag paper is glued with anadhesive, in particular a maltodextrin adhesive. Like the paper itself,maltodextrin adhesives are water-soluble so as to enable a filled bagaccording to the invention to be, for instance, directly introduced intoa concrete mixer along with the cement or mortar contained therein, andhence free from residue or just by leaving statistically finelydistributed cellulose fibers in the construction material to beproduced.

In order to achieve an even higher strength and, in particular, for bagswith heavy contents or those dimensioned to be especially large, thepaper bag according to the invention is further developed such that itis comprised of a multi-ply bag paper.

In order to achieve barrier properties, the paper bag can be furtherdeveloped, to the effect that a layer of water-soluble polymers such aspolyvinyl alcohol or polyether is additionally applied on at least onepaper ply.

In the following, the invention will be explained in more detail by wayof exemplary embodiments.

EXAMPLE 1

Sample bags are produced from a water-soluble unbleached bag paperaccording to the invention and filled with cement. The bags filled withcement are closed and immediately used in a concrete mixer while beingsubjected to a conventional cement mixing process for 5 minutes and 10minutes, respectively, whereupon the results were checked.

Used bag paper: non-bleached water-soluble sack/bag Kraft paper with abasis weight according to ISO 536 of 70 g/m².

The cellulose fibers used in the bag paper comprise long fibers ofspruce wood that was subjected to an oxidation treatment. The paper doesnot contain any sizing agent, yet 0.3 weight-% polyalkyleneglycol ethers(fatty alcohol alkoxylate) with MW 588 g/mol was contained as anon-ionic surfactant. The tensile strength of the paper according to ISO1924-3 was 5.7 kN/m² in the machine direction and 4.2 kN/m² in thedirection transverse to the machine running direction, thuscorresponding to a tensile strength index of 1.36. From this base paper,three different cement sacks were produced, wherein gluing of the sackswas performed using a maltodextrin adhesive. The first cement sack wascomprised of two plies of the above-defined unbleached Kraft paperaccording to the invention without intermediate ply, the second cementsack comprised a ply of unbleached Kraft paper according to theinvention and a second ply of unbleached Kraft paper having a polyvinylalcohol coating. The coating had a thickness of 30 μm. The third cementsack produced comprised a first layer of unbleached, unsized,water-soluble Kraft paper according to the invention, a bare PVA layerhaving a thickness of 38 μm, and a second layer of unsized, unbleachedKraft paper according to the invention. Each of the paper layers in allthree cement sacks had a basis weight of 70 g/m².

The sacks were examined with respect to non-dissolved paper pieces inthe concrete mixer after mixing times of 5 minutes and 10 minutes,respectively, wherein it was found that, as regards sack 1, only verysmall paper pieces were visible on a sieve at the end of 5 minutes,which hardly changed even after 10 minutes.

With sack 2, a plurality of small paper pieces can still be observedafter 5 minutes, which have been significantly reduced after 10 minutes.

With sack 3, a plurality of small paper pieces are still visible after 5minutes, wherein many paper pieces are still visible even after 10minutes.

It is thus to be noted that a paper bag made of unsized, unbleached bagpaper according to the invention, which paper bag has no additionalcoating, is able to dissolve almost completely within 5 minutes. Papersacks 1 to 3 were subsequently filled with cement and subjected to aplurality of handling operations such as rapid filling, closing,stacking, transport, i.e. vibrations, throwing on the ground and thelike, wherein it turned out as a result that paper sack 2 was able towithstand throwing on the ground from a height of 1 meter. Paper sack 1was stable in all handling activities, yet burst open when thrown downfrom a height of 1 meter. Paper sack 1 withstood dropping from a heightof 0.5 meter. The properties of paper bag 3 were comparable to those ofpaper bag 1.

1. A water-soluble unbleached bag paper comprising lignin-containingcellulose fibers as main component and one or more additive(s),characterized in that at least one surfactant is contained as saidadditive, that the cellulose fibers are used as oxidatively treatedunbleached softwood fibers, and that the bag paper has a relative wetstrength of less than 6% after a 5-minute wetting time.
 2. A bag paperaccording to claim 1, characterized in that between 0.1% and 2%surfactant is contained.
 3. A bag paper according to claim 1 or 2,characterized in that the at least one surfactant is selected from thegroup of non-ionic, anionic, cationic or amphoteric surfactants, fattyalcohols, fatty alcohol polylglycol ethers, fatty alcohol sulfates,fatty alcohol, ether sulfates, methyl ether sulfates, fatty alcoholethoxylates or phenol ethoxylates.
 4. A bag paper according to claim 1,2 or 3 characterized in that it shows a absorbency corresponding to aK-value of more than
 5000. 5. A bag paper according to any one of claims1 to 4, characterized in that a dry-strength agent is additionallycontained.
 6. A bag paper according to any one of claims 1 to 5,characterized in that the dry-strength agent is selected from starcheslike cationic potato starch, corn starch, tapioca starch.
 7. A bag paperaccording to any one of claims 1 to 6, characterized in that thecellulose fibers are used as oxidatively treated, unbleached softwoodfibers.
 8. A bag paper according to claim 7, characterized in that thecellulose fibers are selected from softwood fibers from conifers, inparticular fibers from spruce wood and/or pinewood.
 9. A bag paperaccording to any one of claims 1 to 8, characterized in that thesoftwood fibers are used as high-consistency-beaten (HC-beaten) and/orlow-consistency-beaten (LC-beaten) softwood fibers.
 10. A bag paperaccording to any one of claims 1 to 9, characterized in that thelongitudinal/transverse ratio of the tensile strength is between 1 and1.8, in particular 1.3.
 11. A bag paper according to any one of claims 1to 10, characterized in that the cellulose fibers are contained asdried, in particular flake-dried, pulp.
 12. A paper bag comprised of thewater-soluble unbleached bag paper according to any one of claims 1 to11, characterized in that the bag paper is glued with an adhesive, inparticular a maltodextrin adhesive.
 13. A paper bag according to claim12, characterized in that it is comprised of a multi-ply bag paper. 14.A paper bag according to claim 12 or 13, characterized in that a layerof water-soluble polymers such as PVA or PE is applied on at least onepaper ply.